Asymmetrical feedback for coordinated transmission systems

ABSTRACT

A method includes, in a mobile communication terminal, receiving from at least first and second base stations, which cooperate in a coordinated transmission scheme, signals that are transmitted over respective first and second communication channels. Respective channel measures are calculated for the communication channels based on the received signals. First and second feedback data, which are indicative of the respective channel measures of the first and second communication channels, are formulated such that the first feedback data has a first data size and the second feedback data has a second data size, different from the first data size. The first and second feedback data are transmitted from the mobile communication terminal to at least one of the base stations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 61/259,595, filed Nov. 9, 2009, whose disclosure isincorporated herein by reference.

FIELD OF THE DISCLOSURE

The present invention relates generally to communication systems, andparticularly to methods and systems for providing feedback regardingcommunication channels.

BACKGROUND

Some Multiple-Input Multiple-Output (MIMO) communication systems usecooperative transmission schemes, in which multiple base stationscoordinate beamforming and precoding decisions with one another.Coordinated transmission is also referred to as coordinated beamformingor Coordinated Multipoint (CoMP). Coordinated transmission isconsidered, for example, for Evolved Universal Terrestrial Radio Access(E-UTRA) systems, also referred to as Long-Term Evolution (LTE), whichare specified by the Third Generation Partnership Project (3GPP).Cooperative beamforming for LTE is described, for example, in documentR1-093488 of the 3GPP Technical Specification Group (TSG) Radio AccessNetwork (RAN), entitled “LTE Spectral Efficiency and IMT-AdvancedRequirements,” Shenzhen, China, Aug. 24-28, 2009, which is incorporatedherein by reference.

Coordinated transmission schemes often use feedback regarding thecommunication channels, which is fed back from the mobile terminals tothe base stations. An example feedback scheme for coordinatedtransmission is described in 3GPP TSG RAN document R1-092634, entitled“CoMP Operation Based on Spatial Covariance Feedback and PerformanceResults of Coordinated SU/MU Beamforming,” Los Angeles, Calif., Jun.29-Jul. 3, 2009, which is incorporated herein by reference.

3GPP TSG RAN document R1-093474, entitled “Coordinated Beamforming withDL MU-MIMO,” Shenzhen, China, Aug. 24-28, 2009, which is incorporatedherein by reference, describes a Multi-User MIMO (MU-MIMO) scheme withcoordinated beamforming, which is based on a long-term wideband transmitcovariance matrix.

CoMP schemes are also considered for LTE-Advanced (LTE-A) systems.Example CoMP schemes for LTE-A, with reference to feedback, aredescribed in 3GPP TSG RAN document R1-093833, entitled “SystemPerformance Comparisons of Several DL CoMP schemes,” Miyazaki, Japan,Oct. 12-16, 2009, which is incorporated herein by reference. 3GPP TSGRAN document R1-093132, entitled “DL performance of LTE-A: FDD,”Shenzhen, China, Aug. 24-28, 2009, which is incorporated herein byreference, describes LTE-A MU-MIMO schemes with CoMP using FrequencyDivision Duplexing (FDD). 3GPP TSG RAN document R1-093109, entitled“Feedback in Support of DL CoMP: General Views,” Shenzhen, China, Aug.24-28, 2009, which is incorporated herein by reference, discussesseveral feedback design options for implementing CoMP in LTE-A systems.

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

SUMMARY

An embodiment that is described herein provides a method used in amobile communication terminal. The method includes receiving from atleast first and second base stations, which cooperate in a coordinatedtransmission scheme, signals that are transmitted over respective firstand second communication channels. Respective channel measures arecalculated for the communication channels based on the received signals.First and second feedback data, which are indicative of the respectivechannel measures of the first and second communication channels, areformulated such that the first feedback data has a first data size andthe second feedback data has a second data size, different from thefirst data size. The first and second feedback data are transmitted fromthe mobile communication terminal to at least one of the base stations.

In an embodiment, formulating the first and second feedback dataincludes including in the first feedback data at least one feedbackparameter that is not included in the second feedback data. In anotherembodiment, formulating the first and second feedback data includesrepresenting the first feedback data at a first quantization, andrepresenting the second feedback data at a second quantization,different from the first quantization. In yet another embodiment,formulating the first and second feedback data includes calculating thefirst feedback data at a first spectral resolution, and calculating thesecond feedback data at a second spectral resolution, different from thefirst spectral resolution.

In a disclosed embodiment, transmitting the first and second feedbackdata includes transmitting the first feedback data at a first updaterate, and transmitting the second feedback data at a second update rate,different from the first update rate. In another embodiment, when thefirst base station is designated as a serving base station via which themobile communication terminal conducts calls, formulating the first andsecond feedback data includes causing the second data size to be smallerthan the first data size. In yet another embodiment, formulating thefirst and second feedback data includes, upon identifying that firstinterference caused by the first base station is stronger than secondinterference caused by the second base station, computing the firstfeedback data at a first data size and computing the second feedbackdata at a second data size, smaller than the first data size.

In some embodiments, formulating the first and second feedback dataincludes computing for the first and second communication channelsrespective first and second channel matrices having respective differentfirst and second ranks. In an embodiment, formulating the first andsecond feedback data includes defining the first feedback data as therespective channel measure of the first communication channel, anddefining the second feedback data as an implicit function of therespective channel measure of the second communication channel. Inanother embodiment, the method includes receiving in an additionalmobile communication terminal a signal from the first base station overa third communication channel, formulating third feedback data for thethird communication channel such that the third feedback data has athird data size that is different from the first data size, andtransmitting the third feedback data from the additional mobilecommunication terminal to at least the one of the base stations.

There is additionally provided, in accordance with an embodiment that isdescribed herein, apparatus including a receiver, a transmitter andprocessing circuitry. The receiver is configured to receive from atleast first and second base stations, which cooperate in a coordinatedtransmission scheme, signals that are transmitted over respective firstand second communication channels. The processing circuitry isconfigured to calculate respective channel measures for thecommunication channels based on the received signals, and to formulatefirst and second feedback data that are indicative of the respectivechannel measures of the first and second communication channels, suchthat the first feedback data has a first data size and the secondfeedback data has a second data size, different from the first datasize. The transmitter is configured to transmit the first and secondfeedback data to at least one of the base stations. In an embodiment, amobile communication terminal includes the apparatus described herein.In another embodiment, a chipset for processing signals in a mobilecommunication terminal includes the apparatus described herein.

There is also provided, in accordance with an embodiment that isdescribed herein, a system that includes at least first and second basestations and a mobile communication terminal. The base stationscooperate in a coordinated transmission scheme and are configured totransmit signals to mobile communication terminals. The mobilecommunication terminal is configured to receive the signals from thefirst and second base stations over respective first and secondcommunication channels, to calculate respective channel measures for thecommunication channels based on the received signals, to formulate firstand second feedback data that are indicative of the respective channelmeasures of the first and second communication channels, and to transmitthe first feedback data at a first data size and the second feedbackdata at a second data size, different from the first data size, to atleast one of the base stations.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a communicationsystem using coordinated transmission and asymmetrical feedback, inaccordance with an embodiment that is described herein; and

FIG. 2 is a flow chart that schematically illustrates a method forcommunication using coordinated transmission and asymmetrical feedback,in accordance with an embodiment that is described herein.

DETAILED DESCRIPTION OF EMBODIMENTS

In some cooperative transmission schemes, base stations collect feedbackfrom mobile communication terminals regarding communication channelcharacteristics, and configure downlink transmissions based on thefeedback. In many practical scenarios, the volume of feedback that istransmitted from various terminals in the network consumes considerableuplink bandwidth, and may therefore degrade system capacity andperformance.

Embodiments that are described hereinbelow provide improved methods andsystems for delivering feedback from mobile communication terminals tobase stations that use cooperative transmission. In these methods andsystems, a given terminal receives downlink signals from multiple basestations over multiple communication channels, and generates feedbackdata for the multiple channels. The terminal transmits the feedback datato at least one of the base stations, typically to a base station thatis defined as the serving base station of the terminal. The basestations configure their downlink transmissions based on the feedbackdata.

In the disclosed embodiments, the terminal formulates the feedback datafor one communication channel with a certain data size, and with adifferent data size for another communication channel. In other words,the terminal formulates the feedback data for at least two of thechannels to differ in data size. Several example techniques forgenerating feedback data with non-uniform, or asymmetrical, data sizeare described herein. The feedback data for different channels maydiffer, for example, in the number or identity of feedback parameters,quantization level, spectral resolution and/or update rate.

In some embodiments, the terminal selects which channels will receivelarger-size feedback data and which channels will receive smaller-sizefeedback data, according to a certain selection criterion. Severalexamples of selection criteria are described herein. Typically, channelswhose impact on the terminal (e.g., interference) is large will receivelarger-size feedback data, and vice versa. In some embodiments,non-uniform data size is applied across different terminals. In otherwords, two terminals may transmit feedback data having different datasizes.

The disclosed techniques enable the terminal to match the data size (andthus the accuracy) of the feedback data per communication channel,rather than having to compromise for a fixed data size for all channels.As a result, highly-accurate feedback can be obtained where needed,while transmitting smaller-size feedback data for the other channels.Thus, the average uplink bandwidth used for feedback transmission isreduced considerably, with little or no degradation in feedbackperformance.

FIG. 1 is a block diagram that schematically illustrates a communicationsystem 20, which uses coordinated transmission and asymmetricalfeedback, in accordance with an embodiment that is described herein.System 20 comprises a mobile communication terminal 24 (also referred toas User Equipment—UE), and two Base Transceiver Stations (BTSs) 28A and28B. UE 24 may comprise, for example, a cellular phone, acommunication-enabled mobile computing device, a cellular adapter for amobile computing device, or any other suitable communication terminal.Although FIG. 1 shows a single UE and two BTSs for the sake of clarity,real-life systems typically comprise multiple UEs and multiple BTSs.

In the present example, system 20 operates in accordance with the 3GPPLong-Term Evolution Advanced (LTE-A) specifications. Alternatively,however, system 20 may operate in accordance with any other suitablecommunication standard or protocol. For example, the disclosedtechniques can also be applied in Wi-Fi systems operating in accordancewith the IEEE 802.11 specifications or in WiMAX systems operating inaccordance with the IEEE 802.16m specifications.

The BTSs in system 20 use a coordinated transmission scheme, in whichthey coordinate their downlink transmissions, and in particularcoordinate their scheduling and beamforming decisions. At a given pointin time, UE 24 receives downlink signals from multiple BTSs. A subset ofthese BTSs (which may comprise all the BTSs that are received by the UEor a partial subset of the received BTSs) is defined as the reportingset of the UE, i.e., as the set of BTSs for which the UE provideschannel feedback.

In UE 24, the downlink signals are received by one or more UE antennas32. Typically, system 20 comprises a MIMO system, meaning that the BTSsand the UE each comprises multiple antennas. Each downlink signal isreceived over a respective communication channel between a certain BTSand the UE. UE 24 comprises a downlink receiver 36, which receives thedownlink signals from the BTSs. Receiver 36 typically down-converts,filters and digitizes the downlink signals. The UE further comprises achannel measure calculation unit 40, which calculates a respectivechannel measure for each communication channel by processing thereceived downlink signals. Unit 40 may calculate various types ofchannel measures that are indicative of the characteristics of therespective channels.

Some channel measures are explicit, i.e., refer to channelcharacteristics irrespective of any specific transmission or receptionscheme. Other channel measures are implicit, i.e., based on certainassumptions regarding the transmission or reception scheme. Examples ofexplicit channel measures comprise channel matrices (e.g., matricesrepresenting the transfer amplitude and phase for different pairs of BTSantenna and UE antenna) and channel covariance matrices (e.g., matricesrepresenting auto- and cross-correlations between the signals receivedvia different pairs of BTS antenna and UE antenna). Examples of implicitchannel measures comprise Preferred Matrix Indices (PMI) and ChannelQuality Indications (CQI), as defined in the E-UTRA specifications. Unit40 may calculate any of these types of channel measures, or any othersuitable type of channel measure. For a MIMO system, each channelmeasure typically comprises multiple parameters.

UE 24 comprises a feedback formulation unit 44, which formulates thechannel measures produced by unit 40 into respective feedback data. Inparticular, unit 44 produces feedback data having non-uniform data size.In other words, the feedback data for one channel may differ in datasize from the feedback data for another channel. Example techniques forformulating the feedback data at a non-uniform data size are describedbelow.

Note that the term “feedback data that differ in data size” refers onlyto non-zero data size, i.e., to those channels (or BTSs) for which theUE provides non-empty feedback. By contrast, a BTS for which the UE doesnot provide feedback (e.g., a BTS that is not in the reporting set ofthe UE) is not considered herein as differing in feedback data size froma BTS for which the UE does provide feedback.

In the description above, UE 24 generates the feedback data in twostages—channel measure calculation followed by feedback dataformulation. In such embodiments, the channel measures are notnecessarily of non-uniform data size. Assignment of different data sizesto different feedback data (i.e., for different channels) is carried outby unit 44. In alternative embodiments, the functions of units 40 and 44are combined in a single unit. In such embodiments, the channel measuresare calculated a-priori at a data size that potentially differs from onechannel to another.

Unit 44 provides the feedback data for the different channels to anuplink transmitter 48. The uplink transmitter converts the feedback datato an analog signal, up-converts the signal to a suitable RadioFrequency (RF) and transmits the RF signal (and thus the feedback data)to at least one of the BTSs. Typically, one of the BTSs is defined asthe serving BTS of the UE, and transmitter 48 transmits the feedbackdata to the serving BTS. The serving BTS distributes the feedback to theother BTSs, and the BTSs use this information for configuring theirdownlink transmissions.

In the example configuration of FIG. 1, UE 24 receives downlink signalsfrom two BTSs 28A and 28B over two communication channels denoted CH1and CH2, respectively. The UE calculates channel measures and formulatesfeedback data for these two channels. In the present example, thefeedback data for CH1 has a different data size relative to the feedbackdata for CH2. BTS 28A is defined as the serving BTS of UE 24, andtherefore the UE transmits the feedback data for both channels to thisBTS. BTS 28A distributes the feedback data to BTS 28B, so that the twoBTSs can configure and coordinate their downlink transmissions based onthe feedback data.

The system configuration shown in FIG. 1 is a simplified exampleconfiguration, which is depicted solely for the sake of conceptualclarity. In alternative embodiments, any other suitable systemconfiguration can be used. For example, UE 24 may receive downlinksignals from any suitable number of BTSs, and produce feedback data forany desired number of channels. In some networks, two or more sets ofequipment (e.g., transceivers and antennas), which serve differentgeographical cells, are collocated with one another. In the context ofthe present patent application, the equipment serving each cell isregarded as a separate BTS. Multiple BTSs serving multiple cells may becollocated in the same location.

The UE configuration shown in FIG. 1 is a simplified exampleconfiguration, which is depicted solely for the sake of conceptualclarity. In alternative embodiments, any other suitable UE configurationcan be used. UE elements that are not necessary for understanding thedisclosed techniques have been omitted from the figure for the sake ofclarity.

The different elements of UE 24, including receiver 36, units 40 and 44and transmitter 48, may be implemented using dedicated hardware, such asusing one or more Application-Specific Integrated Circuits (ASICs),Field-Programmable Gate Arrays (FPGAs) or other suitable hardwaredevices. Alternatively, some UE elements may be implemented usingsoftware configured to run on a processor device, or using a combinationof hardware and software elements. When certain UE functions areimplemented using a processor, the processor is programmed in softwareto carry out the functions described herein, although it too may beimplemented on dedicated hardware. The software may be downloaded to theprocessor in electronic form, over a network, for example, or it may,alternatively or additionally, be provided and/or stored onnon-transitory tangible media, such as magnetic, optical or electronicmemory. In some embodiments, some or all of the elements of UE 24 may befabricated in a chip-set.

Feedback formulation unit 44 may formulate the feedback data fordifferent channels at different data sizes in various ways. Moreover,unit 44 may use various criteria to select the appropriate data size foreach channel. Generally, the accuracy of the feedback data increaseswith data size. Thus, modifying the feedback data size can tradefeedback accuracy for uplink throughput, and vice versa.

In some embodiments, unit 44 formulates the feedback data for thechannel received from the serving BTS at a larger data size, incomparison with the channels received from other BTSs. In the exampleconfiguration of FIG. 1, unit 44 formulates the feedback data for CH1 tohave a larger data size than the feedback data for CH2. Since the impactof the serving BTS on the UE performance is stronger than the impact ofother BTSs, providing higher-accuracy feedback for the serving BTS andlower-accuracy feedback for the other BTSs increases the feedbackefficiency.

In some embodiments, feedback formulation unit 44 selects theappropriate feedback data size based on the level of interference causedby the different BTSs. For example, unit 44 may produce large-size(high-accuracy) feedback data for BTSs that cause strong interference tothe UE, and vice versa. Additionally or alternatively, in an embodiment,unit 44 selects the feedback data size for the different downlinkchannels in accordance with any other suitable criterion.

In an embodiment, unit 44 generates feedback data at a non-uniform datasize using various techniques. The description that follows refers tothe feedback data for channels CH1 and CH1 of FIG. 1, and presentsexample techniques for formulating the feedback data to differ in datasize. The feedback data for CH1 is referred to as FB1, and the feedbackdata for CH2 is referred to as FB2, respectively. In the presentexample, both FB1 and FB2 are transmitted to BTS 28A, the serving BTS ofUE 24. Since BTS 28A is the serving BTS, FB1 is formulated to have alarger data size than FB2. Generally, however, the techniques describedbelow can be used in any other suitable system configuration.

In an example embodiment, FB1 and FB2 each comprise one or more feedbackparameters. UE 24 formulates FBI to differ from FB2 in the number and/oridentity of the parameters. Consider, for example, an embodiment inwhich each feedback data comprises a covariance matrix for therespective channel. In an example embodiment, FB1 comprises a certainnumber of eigenvectors (of the covariance matrix for CH1) and FB2comprises a smaller number of eigenvectors (of the covariance matrix forCH2). The number of reported eigenvectors is sometimes referred to asthe rank of the feedback data. In an example embodiment, the number ofeigenvectors in FB1 is up to the maximum rank of transmission for theUE, and the number of eigenvectors in FB2 is 1 or alternatively 2. In atypical embodiment for a 2-antenna UE, FB1 comprises two eigenvectorsand FB2 comprises a single eigenvector.

Transmitting different numbers of feedback parameters is useful, forexample, in a Coordinated Beamforming (CB) mode of operation, in whichinterfering BTSs refrain from transmitting to a given UE at a given timein order to reduce interference. Nevertheless, this technique can alsobe used in other coordinated-transmission operational modes, as well. Inalternative embodiments, UE 24 may configure FB1 and FB2 to differ inthe number and/or identity of any other suitable feedback parameters.

In another embodiment, UE 24 formulates FB1 to have a finer quantizationlevel than FB2. In some embodiments, UE 24 selects the feedback data(e.g., channel matrix or covariance matrix) from a predefined set ofpossible values, referred to as a codebook. In these embodiments, UE 24selects FB1 from a larger codebook than the codebook from which FB2 isselected.

Consider, for example, a coherent Joint Processing (JP) mode ofoperation, in which two or more BTSs jointly generate a giventransmission beam toward the UE. When using this mode, the signalreceived at the UE is often predominated by the one or two strongestBTSs. In a scenario of this sort, transmitting feedback data usingdifferent-size codebooks for different BTSs is effective, since itprovides considerable bandwidth reduction with little or no degradationin feedback efficiency. In an example embodiment, the UE selects thefeedback data for the strongest BTS from a codebook whose size isdenoted B, the feedback data for the next-strongest BTS from a codebookwhose size is denoted B/2, and so on. In an example embodiment, B=2^(m),wherein possible values of m are 4, 5 and 6. Alternatively, any othersuitable codebook sizes can be used. Although the example above refersto JP mode of operation, this technique can also be used in othercoordinated-transmission operational modes, as well.

An example of using different feedback parameters for different channelsis the use of feedback based on implicit and explicit channel measures.Explicit channel measures, as defined above, refer to channelcharacteristics irrespective of any specific transmission or receptionscheme. Implicit channel measures, on the other hand, are based oncertain assumptions regarding the transmission or reception scheme.Explicit channel measures comprise, for example, channel matrices orcovariance matrices. Implicit channel measures comprise, for example,PMI or CQI. In some embodiments, UE 24 formulates FB1 (for the servingBTS) based on one or more explicit channel measures, and FB2 based onone or more implicit channel measures. In alternative embodiments, UE 24formulates FB1 based on one or more implicit channel measures, and FB2based on one or more explicit channel measures.

In some embodiments, UE 24 formulates FB1 to have a finer quantizationlevel than FB2 by representing FB1 at a finer numerical precision thanFB2. For example, the feedback parameters in FB1 can be representedusing a larger number of bits than FB2. In alternative embodiments, theUE may use any other suitable scheme for formulating the feedback datafor different channels to differ in quantization level.

In some embodiments, UE 24 formulates FB1 to have a finer spectralresolution than FB2. In these embodiments, UE 24 estimates the feedbackdata for each channel (e.g., channel matrix or covariance matrix) in acertain number of frequency sub-bands. The UE transmits FB1 for a largernumber of sub-bands than FB2. In other words, each feedback parameter inFB1 corresponds to a narrower sub-band than the respective parameter inFB2. Therefore, FB1 is typically more accurate than FB2, at the expenseof larger data size. In the present example, the feedback data for theserving BTS is transmitted at a finer spectral resolution than thefeedback data for other BTSs. Consider, for example, a total bandwidthof 10 MHz that comprises fifty resource blocks. In an exampleembodiment, FB1 comprises ten values for ten respective sub-bands, eachsub-band comprising five resource blocks, and FB2 comprises a singlevalue for the entire bandwidth (fifty resource blocks). Alternatively,however, UE 24 can formulate the feedback data to differ in spectralresolution using any other suitable criterion.

In some embodiments, UE 24 updates FB1 at a faster update rate than FB2.Each update may be of the same data size or of non-uniform data size.Nevertheless, because FB1 is updated more frequently, the data size ofFB1 over a certain time period is larger than the data size of FB2. Inan example embodiment, FB1 is updated every 5-50 mS, and FB2 is updatedevery 50 mS-1 S. Alternatively, any other suitable update rates can beused.

UE 24 may use a higher update rate for the feedback data regardingstronger BTSs, and lower update rates for the feedback data regardingweaker BTSs, or vice versa.

In some embodiments, UE 24 sets a different update rate for differentfeedback parameters in the feedback data of a given channel. Forexample, when the feedback data comprises a channel matrix, the UE mayupdate the magnitudes of the matrix elements (which sometimes varyrapidly) at a relatively fast update rate, and the phases of the matrixelements (which often vary slowly) at a slower rate. This differentialselection of update rate may also vary from one channel to another.

In some embodiments, FB1 may differ from FB2 in a combination of two ormore of the above properties (e.g., number or identity of parameters,quantization level, spectral resolution and/or in update rate), or inany other suitable property.

FIG. 2 is a flow chart that schematically illustrates a method forcommunication using coordinated transmission and asymmetrical feedback,in accordance with an embodiment that is described herein. The methodbegins at a reception operation 60, with UE 24 receivingcoordinated-transmission downlink signals from two or more BTSs overrespective communication channels. At a channel measurement operation64, unit 40 in UE 24 calculates channel measures (e.g., channel orcovariance matrices) for the respective communication channels.

At a feedback formulation operation 68, unit 44 in UE 24 formulates thefeedback data for each channel based on the respective channel measure.In particular, unit 44 formulates the feedback data for at least two ofthe channels to differ in data size, as explained above. At a feedbacktransmission operation 72, transmitter 48 in UE 24 transmits thefeedback data for the multiple channels to at least one of the BTSs,typically to the serving BTS. At a transmission coordination operation76, the BTSs coordinate their downlink transmissions to UE 24 (and/or toother UEs) based on the feedback data.

In the embodiments described above, the UE formulates different-sizefeedback data for different communication channels (i.e., for differentBTSs). Additionally or alternatively, non-uniformity in feedback datasize can be applied across different UEs. In other words, two (or more)different UEs can formulate the feedback data for a given BTS to differin data size. Any of the techniques described above (e.g., feedback datathat differs in the number or identity of parameters, in quantizationlevel, in spectral resolution and/or in update rate) can be used.

Although the embodiments described herein mainly address generation ofchannel feedback data from mobile communication terminals to basestations at a non-uniform data size, the methods and systems describedherein can also be used in other applications in which multipletransmitters coordinate their transmission with one another, such as inWi-Fi and WiMAX systems.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsub-combinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

1. A method, comprising: in a mobile communication terminal, receivingfrom at least first and second base stations, which cooperate in acoordinated transmission scheme, signals that are transmitted overrespective first and second communication channels, and calculatingrespective channel measures for the communication channels based on thereceived signals; formulating first and second feedback data that areindicative of the respective channel measures of the first and secondcommunication channels, such that the first feedback data has a firstdata size and the second feedback data has a second data size, differentfrom the first data size; and transmitting the first and second feedbackdata from the mobile communication terminal to at least one of the basestations.
 2. The method according to claim 1, wherein formulating thefirst and second feedback data comprises including in the first feedbackdata at least one feedback parameter that is not included in the secondfeedback data.
 3. The method according to claim 1, wherein formulatingthe first and second feedback data comprises representing the firstfeedback data at a first quantization, and representing the secondfeedback data at a second quantization, different from the firstquantization.
 4. The method according to claim 1, wherein formulatingthe first and second feedback data comprises calculating the firstfeedback data at a first spectral resolution, and calculating the secondfeedback data at a second spectral resolution, different from the firstspectral resolution.
 5. The method according to claim 1, whereintransmitting the first and second feedback data comprises transmittingthe first feedback data at a first update rate, and transmitting thesecond feedback data at a second update rate, different from the firstupdate rate.
 6. The method according to claim 1, wherein, when the firstbase station is designated as a serving base station via which themobile communication terminal conducts calls, formulating the first andsecond feedback data comprises causing the second data size to besmaller than the first data size.
 7. The method according to claim 1,wherein formulating the first and second feedback data comprises, uponidentifying that first interference caused by the first base station isstronger than second interference caused by the second base station,computing the first feedback data at a first data size and computing thesecond feedback data at a second data size, smaller than the first datasize.
 8. The method according to claim 1, wherein formulating the firstand second feedback data comprises computing for the first and secondcommunication channels respective first and second channel matriceshaving respective different first and second ranks.
 9. The methodaccording to claim 1, wherein formulating the first and second feedbackdata comprises defining the first feedback data as the respectivechannel measure of the first communication channel, and defining thesecond feedback data as an implicit function of the respective channelmeasure of the second communication channel.
 10. The method according toclaim 1, and comprising, in an additional mobile communication terminal,receiving a signal from the first base station over a thirdcommunication channel, formulating third feedback data for the thirdcommunication channel such that the third feedback data has a third datasize that is different from the first data size, and transmitting thethird feedback data from the additional mobile communication terminal toat least the one of the base stations.
 11. Apparatus, comprising: areceiver, which is configured to receive from at least first and secondbase stations, which cooperate in a coordinated transmission scheme,signals that are transmitted over respective first and secondcommunication channels; processing circuitry, which is configured tocalculate respective channel measures for the communication channelsbased on the received signals, and to formulate first and secondfeedback data that are indicative of the respective channel measures ofthe first and second communication channels, such that the firstfeedback data has a first data size and the second feedback data has asecond data size, different from the first data size; and a transmitter,which is configured to transmit the first and second feedback data to atleast one of the base stations.
 12. The apparatus according to claim 11,wherein the processing circuitry is configured to include in the firstfeedback data at least one feedback parameter that is not included inthe second feedback data.
 13. The apparatus according to claim 11,wherein the processing circuitry is configured to represent the firstfeedback data at a first quantization, and to represent the secondfeedback data at a second quantization, different from the firstquantization.
 14. The apparatus according to claim 11, wherein theprocessing circuitry is configured to calculate the first feedback dataat a first spectral resolution, and to calculate the second feedbackdata at a second spectral resolution, different from the first spectralresolution.
 15. The apparatus according to claim 11, wherein theprocessing circuitry is configured to generate the first feedback dataat a first update rate, and to generate the second feedback data at asecond update rate, different from the first update rate.
 16. Theapparatus according to claim 11, wherein, when the first base station isdesignated as a serving base station via which the receiver and thetransmitter conduct calls, the processing circuitry is configured tocause the second data size to be smaller than the first data size. 17.The apparatus according to claim 11, wherein the processing circuitry isconfigured to formulate the first and second feedback data by computingfor the first and second communication channels respective first andsecond channel matrices having respective different first and secondranks.
 18. The apparatus according to claim 11, wherein the processingcircuitry is configured to define the first feedback data as therespective channel measure of the first communication channel, and todefine the second feedback data as an implicit function of therespective channel measure of the second communication channel.
 19. Amobile communication terminal comprising the apparatus of claim
 11. 20.A chipset for processing signals in a mobile communication terminal,comprising the apparatus of claim
 11. 21. A system, comprising: at leastfirst and second base stations, which cooperate in a coordinatedtransmission scheme and are configured to transmit signals to mobilecommunication terminals; and a mobile communication terminal, which isconfigured to receive the signals from the first and second basestations over respective first and second communication channels, tocalculate respective channel measures for the communication channelsbased on the received signals, to formulate first and second feedbackdata that are indicative of the respective channel measures of the firstand second communication channels, and to transmit the first feedbackdata at a first data size and the second feedback data at a second datasize, different from the first data size, to at least one of the basestations.